It is also known to fold an optical cavity in the sense that at least one mirror is arranged in the optical cavity in order to guide light radiation emitted by a light source along an indirect path resulting from successive reflection(s) on the mirror or mirrors.
An optical cavity known from the prior art, particularly from the document DE 4102146, includes an elliptical mirror intended to reflect light radiation emitted by a light source. The light source is arranged at a focal point of the elliptical mirror, and a detector is arranged at the other focal point of the elliptical mirror. Thus, if the geometric optics model is considered, the light rays emitted by the light source pass through the optical cavity twice.
If Cmin is denoted as the minimum concentration of gas that can be detected, and Cmax is denoted as the maximum concentration of gas that can be detected, it is possible to show that Cmin and Cmax satisfy, for such an optical cavity of the prior art, the following relations:
            C      min        =          1              α        ⁢                                  ⁢        l        ⁢                              P            0                    ɛ                ⁢                  exp          ⁡                      (                                          -                β                            ⁢                                                          ⁢              l                        )                                          C      max        =                            log          ⁡                      (                                          P                0                            ɛ                        )                                    α          ⁢                                          ⁢          l                    -              β        α            
where:                α (in ppm−1 m−1) is the absorptivity of the gas at the considered wavelength of the light radiation,        β is an attenuation coefficient linked to the intrinsic losses of the optical cavity, without gas,        I (in m) is the interaction length between the gas and the light within the optical cavity,        P0 is the intensity of the light radiation emitted by the source,        ε is the noise of the detector.        
Such a cavity of the prior art is not entirely satisfactory in terms of extent of the detectable concentration area. A spectroscopic sensor is sought which has the widest possible detectable concentration area, i.e. a range [Cmin, Cmax] that is as dynamic as possible.